The present invention relates to a semiconductor device and a manufacturing method thereof, which are favorably fitted to a metal-oxide-semiconductor field-effect transistor.
In a metal-oxide-semiconductor field-effect transistor (hereinafter also referred to as xe2x80x9cMOSFETxe2x80x9d) used in a conventional semiconductor device, a polysilicon film in which impurities are deeply doped is used as a material for a gate electrode. For example, in a manufacturing process technique used in manufacturing a CMOS circuit (Complimentary MOSFET circuit), in order to balance the performance characteristic of the circuit, n-type polysilicon, in the case of n-channel MOSFET (NMOS), and p-type polysilicon, in the case of p-channel MOSFET (PMOS) are used as a material for a gate electrode. Further, for the purpose of lowering the resistance of a gate electrode, a structure to form a transition metal silicide film on the surface of the gate electrode is used.
In this case, however, the work function of an n-type polysilicon film is 4.15 eV and that of a p-type polysilicon film is 5.25 eV, which results in a value largely deviating from the intrinsic mid gap energy of silicon, 4.61 eV. Such large deviation of the value causes an increase in the absolute value of the flat-band voltage VF2 upon considering a MOS capacitor consisting of a laminated structure of a metal, an insulated film and a semiconductor (Signs are different between NMOS and PMOS). Thus, an optimum value * of impurity concentration in the MOSFET channel for controlling the threshold value Vth is shifted close to the value of high concentration.
In such channel of high concentration, scattering by impurities has a large influence, which invites the deterioration of carrier mobility in the channel. Namely, this indicates the deterioration of the current driving ability of MOSFET and gives a material influence on the response characteristic of the circuit.
In order to solve such problem, various gate electrode materials having a work function are being suggested. xe2x80x9cTsu-Jae King and othersxe2x80x9d (IEDM Technical Digest 1990, page 253) and Japanese Patent Laid-Open Publication Hei 5-235335, for example, suggest a structure using an SiGe alloy film as a material for a gate electrode, and xe2x80x9cJeong-Mo Hwang and otherxe2x80x9d(IEMD Technical Digest 1992, page 345) suggests a structure using a TiN film.
FIG. 8 indicates the first example of related arts using an SiGe alloy film for a gate electrode. Explaining FIG. 8, it indicates a structure in which an NMOS transistor 20 and a PMOS transistor 21 are formed on a substrate 1, and an n-type polycrystalline SiGe film 30 and a p-type polycrystalline SiGe film 31 are respectively deposited on a gate oxide 2. Further, a low resistance conductive film for lowering the resistance is provided on the SiGe film 30. Upon using such SiGe alloys as a gate electrode material, a work function can be shifted closer to the intrinsic aid gap energy of silicon in the proportion of germanium atoms contained in the silicon.
Further, Reference no. 5 indicates a source and drain region, Reference no. 22 an n-well region and Reference No. 23 an element separation oxide.
In the aforementioned related art example, however, a substantial improvement of characteristics can only be expected in the PMOS transistor 21. This derives from a physical phenomenon that changes in a band structure in SiGe alloy can be mainly recognized only in the valence band side. That is, the work function of the p-type polycrystalline SiGe film 31 can be controlled by mixing germanium, however, the n-type polycrystalline SiGe film 30 is no more effective than expected.
FIG. 9 indicates an example of related arts using a TiN film for a gate electrode. In FIG. 9, the components identical to those in FIG. 8, are donated by the same reference numerals and the detailed explanation thereof is abbreviated. As in FIG. 8, FIG. 9 indicates a structure in which the NMOS transistor 20 and the PMOS transistor 21 are formed on a substrate 1, and a TiN film 32 is formed on the gate oxide 2. Further, the low resistance conductive film 4 is also provided on the TiN film 32 as provided in the first related art example.
This work function of such TiN film as described in xe2x80x9cJeong Mo Hwang and othersxe2x80x9d (IEDM Technical Digest 1992, page 345), is 4.7 to 4.8 eV, which is close to the intrinsic mid gap energy of silicon, 4.61 eV, and is more effective.
In this case, however, as the work function of a gate electrode is uniquely determined, there is a problem that a little unbalance arises as to the characteristics of the NMOS transistor and the PMOS transistor. Further upon adopting the aforementioned low resistance conductive film 4, the dispersion in work functions is caused by a conductive film formation process, therefore, there is also a drawback that the process conditions should be strictly controlled.
The present invention is made in view of the above problems, and prevents the deterioration of carrier mobility in a channel in a semiconductor, especially an NMOS transistor and a PMOS transistor, and provides a semiconductor device having a high current driving ability and a manufacturing method thereof.
In order to solve this problem, the present invention, in a metal-oxide-semiconductor field-effect transistor formed on a silicon substrate, provides a semiconductor device in which a gate electrode of such transistor is formed with a germanium film.
The germanium film can be a single-crystalline germanium film, a polycrystalline germanium film or an amorphous germanium film. Further, p-type impurities can be doped into the germanium film.
The gate electrode can comprise a multi-layer structure which includes a germanium film and a low resistance conductive film.
Further, the low resistance conductive film can include a transition metal, a transition metal silicide or a transition metal nitride film, or a combination thereof.
Such multi-layer structure can also be provided with a polysilicon film in between a germanium film and a low resistance conductive film.
The present invention provides a semiconductor device having an n-channel metal-oxide-semiconductor field-effect transistor and a p-channel metal-oxide-semiconductor field-effect transistor which complement each other, wherein a gate electrode of each of the transistors comprises a single-crystalline germanium film, a polycrystalline germanium film or an amorphous germanium film in which p-type impurities are doped.
The present invention also provides a manufacturing method of a semiconductor device comprising a step of forming a gate oxide on a semiconductor substrate; a step of forming a germanium film on the gate oxide; a step of doping p-type impurities into the germanium fill and then patterning such germanium film to form a gate electrode and a step of forming a source and drain region by using the gate electrode as a mask.
The step of forming the gate electrode can comprise a step of forming a polysilicon film on the germanium film, a step of forming a transition metal on the polysilicon film and a step of annealing the polysilicon so that a part or all of such polysilicon becomes a transition metal silicide.
The step of forming the gate electrode can include a step of forming a metal transition film or a metal transition nitride film on the germanium film.
The step of doping the p-type impurities can be used by a CVD method.
Furthers the step of doping the p-type impurities is used by an ion implantation method.
Furthermore, the present invention provides a manufacturing method of a semiconductor device comprising a step of forming a gate oxide on a semiconductor substrate, a step of forming a germanium film on the gate oxide, a step of doping p-type impurities into the germanium film and patterning such germanium film to form a gate electrode, a step of forming a source and drain region by using the gate electrode as a mask, a step of forming a spacer on both ends of the gate electrode, and a step of forming a transition metal film on the gate electrode and the source and drain region and a step of annealing the transition metal to form a transition metal silicide.